Method of manufacturing oriented grain magnetic steel sheets

ABSTRACT

METHOD OF ELABORATING METAL SHEETS FOR MAGNETIC APPLICATIONS AND MORE PARTICULARLY SHEETS HAVING A SINGLE OR DOUBLE GRAIN ORIENTATION, WHICH CONSISTS IN ELABORATING A CARBON STEEL HAVING A VERY LOW SULFUR CONTENT, I.E. LESS THAN .005% AND PREFERABLY LESS THAN .003%, REHEATING THIS METAL IN STEELWORK FURNACE AND STRONGLY OXIDIZING THE METAL IN A CONTROLLED MANNER, WHILE AVOIDING ANY RESULFURIZATION, RECARBURIZING THE STEEL IN THE LADLE AND IN VACUUM TO OBTAIN VERY LOW CARBON CONTENTS, PREFEREABLY LESS THAN .005% C. SUBSEQUENTLY ADDING THE NECESSARY ALLOYING ELEMENTS, SILICON AND/OR ALUMINIUM, HOMOGENIZING THIS ADDITION BY STIRRING, CASTING INTO INGOT MOULDS, PREFERABLY WITHOUT AMBIENT AIR CONTACT, THE EXTRA-MILD STEEL THUS OBTAINED, AND FINALLY ROLLING THE RESULTING INGOTS TO CONVERT THEM INTO SHEETS HAVING A SINGLE-OR DOUBLE GRAIN STRUCTURE ORIENTATION.

United States Patent C) US. Cl. 148-111 1 Claim ABSTRACT OF THE DISCLOSURE Method of elaborating metal sheets for magnetic applications and more particularly sheets having a single or double grain orientation, which consists in elaborating a carbon steel having a very low sulfur content, i.e. less than .005% and preferably less than .O03%, reheating this metal in a steelwork furnace and strongly oxidizing the metal in a controlled manner, while avoiding any resulfu-rization, recarburiz'ing the steel in the ladle and in vacuum to obtain very low carbon contents, preferably less than .005 C, subsequently adding the necessary alloying elements, silicon and/or aluminium, homogenizing this addition by stirring, casting into ingot moulds, preferably without ambient air contact, the extra-mild steel thus obtained, and finally rolling the resulting ingots to convert them into sheets having a singleor double grain structure orientation.

This is a continuation of application 'Ser. No. 823,526, filed May -9, 1969, and now abandoned.

The present invention is directed to a method of manufacturing steel sheets for magnetic applications, and more particularly (110) [(001)] cubes-on-edge sheets and (100) [(001)] flat-cube sheets. In this description, the grain orientation is defined by reference to Millers numbers.

It is known that these grain orientations, very useful for all applications wherein the material is utilized as a magnetic circuit, are particularly difiicult to obtain. Various methods have already "been proposed to this end, but so far they either gave irregular results or were abnormally expensive to carry out.

The various hitherto known methods of manufacturing oriented-grain sheets comprise as a rule the stepinserted at a predetermined phase of the manufacturing process-of decarburizing the sheets for example in a wet reducing atmosphere at about 800 C. during a few minutes, said manufacturing process comprising the elaboration or making of the metal itself, and a sequence of hotand cold-rolling steps, the cold-rolling passes being separated from each other by suitable chemical and heat treatments.

Another known process utilizes alloys derived from iron previously decarburized in the solid state, wherein the alloy is remelted in an electric arc furnace under slag in the presence of a silicon and/or aluminium additive.

Now the applicants found and proved that it was more advantageous to start from a very low-carbon steel decarburized in the liquid state.

In a prior application the applicants described a method of manufacturing steel sheets having an oriented grain structure, wherein an extra-mild steel is elaborated in a steelwork furnace, subsequently decarburized in the liquid state in the ladle, in vacuum, before receiving the alloying elements (silicon and/or aluminium), then cast into ingot moulds, whereafter the resulting ingots are subjected to a rolling and heat-treatment cycle differing according as the desired end products are single-orientation sheets of the cubes-on-edge type, i.e. with a [(001)] orientation, or double-orientation sheets of the flat-cube type, i.e. with a (100) [(001)] orientation.

It is the essential object of the present invention to provide a method of producing sheets having a single or double orientation made from very low-carbon and very low-sulfur steel.

The method according to this invention consists in preparing a carbon steel having a very low sulfur content .005%, and preferably .003% reheating the metal in a steelwork furnace and then strongly oxidize the steel in a controlled manner while preventing any re-sulfurization thereof, and subsequently decarburizing the steel in the ladle and in vacuum until very low carbon contents, preferably inferior to .005 are obtained, then adding the necessary alloying elements (silicon and/or aluminium), homogenizing this addition by stirring the molten mass either by injecting a neutral gas such as argon therein, or by using electromagnetic means, or any other suitable means, casting the extra-mild steel thus obtained into ingot moulds preferably under shelter from the surrounding atmosphere, and finally rolling the resulting ingots to convert same into sheets having a sin-gleor doublegrain orientation.

The very low sulfur content .005% and preferably .003%) of the initial carbon steel may be obtained by resorting to any one of the following procedures:

(1) By refining in the electric furnace under reducing slag a relatively carburized steel in order to obtain a carbon content of less than 300%, which may even be reduced to the extra-mild grade containing about 020% C, and a sulfur content inferior to 020%, this metal being subsequently cast with its reducing slag into a first ladle, the entire molten mass being then transferred into another ladle in order properly to exhaust the de-sulfurizing capacity of the reducing slag. Thus, the desired sulfur content, approximately 003%, is obtained. The steel is substantially cooled but this is not detrimental since it is not cast directly into ingot moulds; on the contrary, this steel is then reheated and oxidized in a steel- Work furnace before being decarburized in vacuum.

(2) By treating in the ladle by means of a desulfurizing basic and exothermic flux, a steel elaborated in the electric furnace or in an open hearth furnace and having a carbon content less than 300%, which may even be reduced to the extra-mild grade containing about 020% C, and a sulfur content of the order of .0l8%. The desulfurization flux may consist for example of a mixture of lime, fluor-spar, sodium fluoride and metallic silicon (or ferro-silicon). Thus, after this treatment in the ladle the sulfur content drops from .018% to about .003%.

(3) The steel is firstly elaborated in an oxygen converter. The liquid cast-iron, before being introduced into the converter, is strongly desulfurized in the ladle, for example until the sulfur content has dropped to about .025%. The steel elaborated in the converter has a sulfur content of the order of .017%, provided of course that the lime itself has a sufliciently low sulfur content. The final desulfurization (to reduce the sulfur content to less than .005%) is subsequently carried out either by liquid reducing treatment as explained in paragraph (1) hereinabove, or by stirring with a desulfurizing flux as set forth in paragraph (2) hereinabove.

In any case the first stirring step with a desulfurizing reducing slag in the ladle may be combined with another stirring step in another ladle by using a desulfurizing flux. In this specific process the temperature of the steel issuing from the furnace or the converter must be high enough to undergo this double treatment without leaving any deposit in the ladle bottom, so that it can subsequently be re-introduced integrally into the furnace for re-heating.

The sheet elaboration method according to this invention further comprises the following steps and features, which may be applied separately or in any desired com- 'bination:

(a) The alloying elements are added not in the ladle but in a non-recarburizing electric furnace;

(b) The alloying elements are added not in the ladle but in an induction furnace;

An electrode consisting of low-carbon, low-sulfur metal obtained as disclosed hereinabove but having received little or none of the alloying elements, is remelted in a furnace of the type comprising an electrode consumable under the slag, the whole or the make-up fraction of the alloying elements being added either to the electrode, which thus becomes a composite electrode, or more simply to the liquid fraction of the ingot;

(d) In the case of the manufacture of sheets having a single-orientation grain structure, of the cubes-on-edge type, i.e. with a (110) [(001)] orientation, the silicon content of the alloying elements ranges from 2 to and is preferably of the order of 3%, and the very low aluminium content is preferably less than 02%;

(e) In the case of the manufacture of sheets having a double-orientation grain structure, of the flat-cube type, i.e. with a (100) [(001)] orientation, the aluminium of the alloying elements ranges from 3 to 4.5%, and the silicon content is extremely low, or alternately the silicon content ranges from .8% to 2%, and is preferably close to 1.3%, the aluminium content ranging on the other hand from 1 to 2.5% and being preferably close to 1.7%.

According to this invention the rolling cycle for converting the resulting ingots into sheets having a doubleorientation grain structure comprises in succession a hotrolling step to reduce the thickness of the initial plate to about .4", a pickling step, a first cold-rolling operation for reducing the thickness to the value of .12" to .16" and preferably to about Vs", followed by an intermediate recrystallization annealing at a temperature of 900 C. to l,050 C. and preferably about 1,000 C., in a dry hydrogen atmosphere, during a few hours, preferably during about one hour, then a second cold-rolling operation to reduce the thickness to a value of .035" to and preferably of the order of .042", followed by an intermediate annealing at the same temperature as before, and still in a dry hydrogen atmosphere, but during only a few minutes, preferably during about minutes, and finally a third cold-rolling step until the thickness is reduced to about .012" to .016", and preferably to about .0138", followed by a final recrystallization annealing step at a temperature ranging from about 1,000 C. to about 1,100" O, and preferably of the order of 1,050 0, still in dry hydrogen atmosphere, during a few hours and preferably during approximately 5 hours.

A modification of the above-disclosed method consists in hot-rolling the ingots to reduce the metal thickness to about .1575" to .2362", pickling the metal, performing a first cold-rolling operation to reduce its thickness to a value of about .0315" to .06" and preferably of the order of .042", the intermediate annealing step, the last coldrolling operation and the final annealing step being performed under the same conditions as before.

In the foregoing the dry hydrogen atmosphere for the 4 annealing treatment is such that its dew point is less than -40 C.

Another modification of the method of this invention with a view to convert the ingots into sheets having an oriented grain structure consists in interposing an intermediate treatment aiming at decarburizing the steel in the solid state either just after the hot-rolling operation and before the pickling step, or on the sheet cold-rolled to the final desired thickness. This alternate form of embodiment is particularly advantageous when the degree of decarburization of the steel in the liquid state, performed in vacuum, is not sufficient.

Finally, this invention is also concerned, by way of new industrial products, with metal sheets for magnetic applications having a flat-cube grain structure according to the [(001)] orientation, obtained by the method disclosed hereinabove and characterized by an anisotropic 'curve, recorded on the torsion balance, showing by way of non-limiting example, a higher maximum of about 160,000 ergs/cubic centimeter and a lower maximum of about 150,000 ergs/ cubic centimeter.

What is claimed is:

1. A method of making steel for forming sheets having magnetic uses, said sheets having a singleor doubleoriented grain structure comprising the steps of elaborating a steel having a carbon content of from 0.3% to about 0.02% and a sulfur content of about 0.018%, subsequently treating said steel in a ladle by means of a basic exothermic desulfurizing flux comprising a powdered mixture of lime, fluor-spar, sodium fluoride and metallic silicon or ferro-silicon in order to obtain a steel having a sulfur content of less than 0.003%, reheating the steel in a steelwork furnace and strongly oxidizing the metal while preventing any resulfurization thereof, subjecting the steel melt in a ladle to a vacuum until the carbon content is reduced to about 0.005%, adding alloying elements to the melt selected from the group consisting of silicon, aluminum and mixtures thereof, homogenizing the mixture by stirring, casting the resulting extra-mild steel into ingot molds and rolling the ingots thus-obtained to sheets having a singleor double-grain orientation.

References Cited UNITED STATES PATENTS 2,573,319 11/1949 Dreyfus et al. 75-49 2,626,863 1/1953 Erasmus 75-58 2,662,819 12/1953 Hofges et al. 75-58 2,915,381 12/1'959 Goerrig et al. 75-58 3,058,857 10/1962 Pavlovic et al. 148-120 3,214,303 10/1965 Fiedler 148-111 3,253,909 5-/ 1966 Bishop, Jr., et al. 75-49 3,278,348 10/ 1966 Foster et al. 148-110 3,279,960 10/1'966 Nishihara et al. 148-120 3,345,219 10/19 67 Detert 148-112 3,522,114 7/ 1970 Knuppel et al. 148-111 L. DEWAYNE RUTL'BDGE, Primary Examiner G. K. WHITE, Assistant Examiner UJS. Cl. X.R. 

